This invention relates to polymeric compositions, in particular to film-forming polymeric compositions based on vinylidene chloride copolymers and to processes for forming polymeric films from such compositions. The films so formed being substantially agglomerate-free and bubble-free.
Copolymers of vinylidene chloride with such copolymerizable monomers as acrylonitrile, vinyl chloride and lower alkyl acrylates have found use in the manufacture of filaments, sheets, tubes, films and extruded and molded shapes. It is known that such polymers are difficult to work with due to their brittleness, relatively poor melt flow and susceptibility to thermal degradation, as evidenced by the development of discoloration and the presence of gas bubbles during the fabrication thereof, particularly during extrusion of film materials therefrom. It has, heretofore, been common practice to incorporate plasticizers into vinylidene chloride polymer compositions to improve their workability. Thus, citric acid esters (see British Pat. No. 739,411), sebacic acid esters (see U.S. Pat. No. 2,604,458) and/or phthalate esters (see British Pat. No. 811,532) have been utilized in combination with vinylidene chloride polymers to produce film materials. Polymer compositions containing such plasticizers in conjunction with various stabilizer systems, e.g., combinations of an epoxidized soybean oil and an oxide of the metals of Group II of the periodic table, such as magnesium oxide, have also been used as disclosed, e.g., in U.S. Pat. No. 3,261,793.
It is known that these plasticized vinylidene chloride polymer compositions are less than wholly satisfactory for a number of reasons, e.g., most of the known plasticizers have poor compatibility with vinylidene chloride polymer compositions and migrate to the surface of articles, such as films, produced therefrom. This result is especially disadvantageous when the polymeric film material is used in food wrapping applications. Further, the presence of such plasticizers often significantly reduces tensile strength and increases gas permeability of polymeric films produced from the vinylidene chloride polymers, particularly permeability to water vapor and air. As such, the addition of plasticizers to vinylidene chloride copolymer compositions to produce polymeric film material is not without problems. However, when film material is produced from normally crystalline vinylidene chloride copolymer compositions without the use of plasticizers, the resultant film material is frequently characterized by the presence of gas bubbles. Gas bubbles are detrimental where clear, continuous film material is required. It is believed that the gas bubbles result when the vinylidene chloride copolymer degrades via dehydrohalogenation to evolve gaseous hydrogen chloride.
A solution to the problem of gas bubbles has been disclosed in U.S. Pat. No. 3,891,598 wherein substantially plasticizer-free extruded film material comprising a blend of a normally crystalline vinylidene chloride interpolymer with small amounts of magnesium oxide was prepared in a process comprising the steps of admixing magnesium oxide in dry powdered form with dry powdered vinylidene chloride polymer to form an admixture; passing the admixture through a screen to remove any agglomerates having a diameter of greater than about 500 microns from the admixture; and fabricating the admixture into film form. A special technique of adding the magnesium oxide to the vinylidene chloride polymer resin was devised to counteract the tendency of magnesium oxide particles to cling to metal surfaces and to thereafter form agglomerates which break off into the extruder melt and result in holes in the film extrudate when the extrudate is formed into a stretched film. The special technique involved conducting the magnesium oxide powder via a flexible tube under the surface of dry vinylidene chloride copolymer particles while such polymeric particles were in motion as in a ribbon or cone blender. Films so produced are often characterized by the presence of small white specks or agglomerates of magnesium oxide and frequently by the presence of small holes.
It is, therefore, the primary object of the present invention to produce vinylidene chloride polymer film materials characterized by reduced permeability to gases such as water vapor and air and which, in addition, can be thermally fabricated, e.g., extruded into film form, in the substantial absence of discoloration, gas bubbles, holes and agglomerates of inorganic particulate additives.
A related object is to produce a vinylidene chloride polymer-magnesium oxide blend which is free-flowing and which is substantially free of agglomerates of magnesium oxide.
A further object is to render the magnesium oxide powder free-flowing thereby negating the tendency of the magnesium oxide to cling to metal surfaces with the consequent formation of agglomerates.
Still another object is to render other inorganic particulate additives, which are not inherently free-flowing and which tend to form agglomerates, generally free-flowing and agglomerate-free.